1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly, to control for conveying a sheet when an image formed on an image bearing member is transferred to the sheet.
2. Description of the Related Art
Conventionally, in an image forming apparatus for forming an image on paper, a resin sheet and the like (hereinafter referred to as a sheet) such as a copying machine, an LBP and a facsimile machine, there has been employed an electrophotographic recording system as a recording system in which a sheet is conveyed to a transferring portion, and an image, which is formed by developing a latent image formed on an image bearing member, is transferred to the sheet.
As an image forming apparatus of this type, for example, there is an image forming apparatus that is constituted as shown in FIG. 5. In the image forming apparatus, a registration roller pair 106, which is a pair of skew feeding correcting rollers constituting conveying means, is disposed on the upstream side of a photosensitive drum 112 functioning as an image bearing member, and a sheet S fed from a sheet containing portion 101 by a pick-up roller 102 abuts a nip portion of the registration roller pair 106 in a stopped state to form a loop, whereby skew feeding of the sheet S is corrected.
Further, in such an image forming apparatus, when it is prepared to form an image after skew feeding is corrected, the registration roller pair 106 is rotated in coincided timing with start of exposure on the photosensitive drum 112 to convey the sheet S to a transferring portion P that is constituted by the photosensitive drum 112 and a transferring roller 113. Consequently, a leading edge position of the sheet S and a leading edge position of a toner image on the photosensitive drum 112 are aligned (hereinafter referred to as registration).
However, in such a structure, a state of a sheet leading edge abutting the nip portion may be different depending on a type of a sheet, a state of curl of the leading edge or the like. In such a case, timing for starting to convey the sheet is varied to some extent. As a result, registration cannot be performed successfully in some cases.
Thus, conventionally, there is an image forming apparatus that is. provided with a sensor 21 functioning as sheet detecting means for detecting the sheet S (hereinafter referred to as a top sensor) disposed between the transferring portion P and the registration roller pair 106 as shown in FIG. 6. In this image forming apparatus, an original image, which has been read by a not-shown image reading portion based on a detection signal from the top sensor 21, is started to be written in the photosensitive drum 112 by a laser unit 22 functioning as exposing means, whereby accuracy of registration is improved.
In this structure, given that L is a developing distance of a drum surface between a drum surface irradiating position (writing position) W, on which a laser beam of the laser unit 22 is irradiated and the transferring portion P, and that M is a distance between a sheet detection position C of the top sensor 21 and the transferring portion P, the top sensor 21 is disposed in a position where a relationship of M greater than L is established such that a toner image on the photosensitive drum 112 can be surely transferred to the sheet S. As an image forming apparatus constructed in this way, for example, there is one disclosed in Japanese Patent Application Laid-open No. 2000-281247.
However, in the conventional image forming apparatus as shown in FIG. 6, when the leading edge of the sheet S is detected by the top sensor 21 and an original image is started to be written in the photosensitive drum 112 based on the detection signal, the distance M must be secured between the top sensor 21 and the transferring portion P such that the relationship of M greater than L is established. As a result, a distance of conveying a sheet between the registration roller pair 106 and the transferring portion P becomes longer than that of the image forming apparatus shown in FIG. 5.
In addition, since the sheet S is caused to stand by at the registration roller pair 106 until preparation for writing the original image in the photosensitive drum 112 is finished, when the distance of conveying a sheet between the registration roller pair 106 and the transferring portion P becomes longer, time required for copying (image forming) an image on a first sheet is extended or the number of copies (prints) per unit time is reduced in continuously feeding sheets and forming images on both sides of the sheet S.
Thus, for example, it is possible to feed the sheet by rotation of the registration roller pair 106 before the preparation for writing an original image in the photosensitive drum 112 is finished. However, particularly in a printer used as outputting means of a personal computer or the like which is an example of the image forming apparatus, it is likely that the preparation is not finished until the sheet S reaches the top sensor 21 if an amount of information of an image is large.
As described above, when the distance between the transferring portion P and the registration roller pair 106 becomes longer, time required for copying an image on a first sheet (first copy) is extended and, as a result, the number of copies per unit time is reduced and productivity falls.
The present invention has been devised in view of the above-mentioned drawbacks, and it is an object of the present invention to provide an image forming apparatus that can improve the productivity.
According to the present invention, there is provided an image forming apparatus including: exposing means for exposing an image bearing member provided in a transferring portion according to image information to form a latent image on the image bearing member; conveying means for conveying a sheet to the transferring portion; and sheet detecting means for detecting the sheet conveyed by the conveying means, which is disposed between the transferring portion and the conveying means, in which the image forming apparatus starts to form a latent image by the exposing means based on the detection of the sheet by the sheet detecting means, and in which the image forming apparatus further includes: control means for judging whether or not it is possible to start exposure by the exposing means before the leading edge of the sheet conveyed by the conveying means reaches the sheet detecting means after the leading edge of the sheet passes through the conveying means and for controlling so that the sheet conveyed by the conveying means is stopped in front of the sheet detecting means until it is determined that it is possible to start exposure by the exposing means.